As packaging boxes in which, for example, cosmetics, medicines, confections, stationary and other daily use products are packaged, plastic boxes are widely used in place of paper boxes formed of conventional thick paper or cardboard. A plastic box is often used as a container that is transparent or gives the impression of transparency, and as a packaging box that allows the contents or decorations inside the box to be seen from the outside. The plastic box is preferably formed into a hexahedral shape by an easily assembling process, for example, by bending a plastic sheet which has been formed into a predetermined shape by cutting, along bend lines and then joining at suitable locations.
When assembling a plastic box into a hexahedral shape, a tubular body opened at the upper and lower ends and with a square cross section is assembled using side walls surrounding four directions, and then the bottom surface portion and the upper surface portion are formed by bending inner flaps and outer flaps, provided so as to stretch to the upper edge and the lower edge of the side walls, thereby closing the square opening. In other words, the opening is closed by bending the inner flaps, arranged on one pair of opposite two sides at the periphery of the square opening, towards the inside, and folding the outer flap, provided with an insertion flap piece and arranged on one side of the other pair of opposite two sides, onto the outer side of the folded inner flaps, while inserting and latching the insertion flap piece into the inside of the opening between the other side of the other pair of opposite two sides and the bent inner flap, thereby closing the opening portion and forming the bottom surface portion and the upper surface portion.
In contrast, because the plastic has elasticity, when assembling a plastic box into a hexahedral shape, each surface of the box body assembled in a solid shape by bending along a bending line tends to curve compared to a paper box. Because of this, even though the insertion flap piece is simply inserted into the inside of the opening to form the bottom surface portion or the upper surface portion, the elasticity of the plastic makes it easy to extract the insertion flap piece from the inside of the opening. Consequently, this plastic box has an objective of preventing the insertion flap piece from being extracted in this manner and is provided with a lock mechanism that functions by forming latching notches on respective end parts of the bend line from the outer flap of the insertion flap piece and then latching the base end parts of the inner flap to these notches.
Because the upper surface portion of the plastic box assembled into a hexahedral shape must mainly open and close to allow items to be placed in and taken out of the box, lock mechanisms have been developed taking into consideration the ease of opening and closing the plastic box (as an example refer to Japanese Utility Model Laid-Open Publication No. Hei 6-42632).